A plasma surface metallurgy technique was used on the surfaces of carbon steel to form W-Mo alloying layer and W-Mo-Y alloying layer. Dynamic behavior of W, Mo and Dy was investigated by origin software, Fick’s second law and Arrhenius formula. Results show that Dy addition increases the diffusion coefficient of W and Mo by 0.94 times and 0.62 times, respectively in the diffusion layer from surface of 0~5 μm while increases the diffusion rate of W and Mo by 2.87 times and 1.07 times, respectively in the diffusion layer from surface of 80 μm. However, the Dy addition decreases the activation energy of diffusion of W by 7.13 kJ/mol and Mo by 5.19 kJ/mol from surface of 5 μm, while it decreases the activation energy of diffusion of W by 32.20 and Mo by 10.18 kJ/mol from surface of 90 μm. Under the same process condition, the main influence factor of diffusion in W-Mo-Dy co-diffusion layer is vacancy concentration. Dy decreases activation energy of diffusion of W and Mo in the subsurface